Passivation of mercury cadmium telluride (HgCdTe) surfaces in exposed junctions, particularly in materials responsive to radiant energy in the 8-12 micrometer wavelength, is a difficult problem. The technology of thick native oxide growth in HgCdTe is not available.
In U.S. Pat. No. 3,799,803, which issued in Mar. 1974, the passivation of a semiconductor surface, namely HgCdTe, is described by means of a hydrogen peroxide rinse to remove contaminants from the surface.
In U.S. Pat. No. 3,845,494, which issued in Oct., 1974, a HgCdTe photovoltaic detector is described covered by a continuous film of material such as a metallic sulfide or selenide for preventing outward diffusion of mercury vapor from the detector. The film or coating which is impervious to mercury may include zinc sulfide, zinc selenide or arsenic pentaselenide.
In U.S. Pat. No. 4,132,999, issued Jan., 1979, a PN junction is formed in HgCdTe material by diffusing mercury through a protective layer of cadmium telluride. A mask of zinc sulfide defines the boundaries of the PN junction formed beneath the layer of cadmium telluride.
In U.S. Pat. No. 4,137,544, issued Jan., 1979, a HgCdTe diode is formed by a ion implanting accepter impurity such as phosphorus, antimony or arsenic into an N-type substrate of HgCdTe. In addition, accumulation is formed at the surface of the HgCdTe substrate to surround the p-type region formed by ion implantation. The ion implantation is performed right through a passivation layer such as anodic oxide. An additional mask is provided above the passivation layer to define the p region. The resulting PN junction is formed within the HgCdTe substrate and extends to the surface which is protected by the passivation layer. The diode is particularly useful for detection of radiation in the range from 8-14 micrometers.
In U.S. Pat. No. 4,170,666, which issued in Oct., 1979, the effective surface recombination velocity of III-V compounds semiconductors is reduced by providing a native dielectric passivation layer on the semiconductor and by inducing a potential in the vicinity of the semiconductor-dielectric interface which repels approaching minority carriers. A layer of gallium arsenide phosphide (GaAsP) is formed by converting the surface of GaAs. GaAsP has a higher energy band gap than GaAs and increases the energy which minority carriers must posses in order to reach the interface of the GaAsP and a gallium phosphorus oxide passivation dielectric.
It is therefore desirable to fabricate PN junctions in HgCdTe substrates with reduced surface leakage current and higher R.sub.0 A products by forming the PN junction underneath the protective surface. One typical prior art solution which is disclosed in U.S. Pat. No. 4,549,195, issued Oct., 1985. The heterojunction semiconductor device has the PN heterojunction between two layers of material each having a different band gap. The edges of the PN heterojunction are buried or concealed below the surface of one of the layers to reduce leakage occurring across the PN heterojunction. More particularly, the layer having the greatest energy bandgap fully covers the boundaries or perimeter of the layer having the lesser energy bandgap.
A disadvantage and limitation of passivation technique of the '195 patent is that a mesa etch of the narrow bandgap materials required prior to growing of the large bandgap layer. The crystal orientation of the sidewalls of the mesa are not oriented properly for epitaxial growth of cadmium telluride thereon. Therefore, in the prior art approach, the junction may not be passivated.